The ability of a fleet of mobile stations, such as service vehicles used in a variety of industries, to transmit radio data messages to a fixed base station over a selected radio channel is well-known in the art. As the number of mobile stations in the fleet, or more specifically, the number of messages transmitted between the mobile stations and the base station increases, a protocol must be employed to ensure the availability of the radio data communications link for priority data exchanges between the mobile stations and the base station. Several existing techniques have been employed to address this problem.
One known method of addressing this problem is to limit the number of mobile stations in a fleet or to increase the number of radio channels assigned to a fleet. However, this solution leads to inefficient use of the radio channel or channels employed, and it is typically not cost effective in practice.
Time division multiplexing is another conventional solution to this problem. In time division multiplexing, each mobile station in the fleet is given a dedicated time slot over a given time interval for communication with the base station. Each mobile station can only communicate to the base station during its dedicated time slot. Time division multiplexing is an effective technique in systems with a large number of mobile stations and a large number of messages which need to be communicated to the base station. However, this technique results in inefficient use of a radio data communications link if a given mobile station does not send a message during its dedicated time slot or fails to fully utilize its time slot for communication. In addition, a given mobile station can only send a limited amount of information during its dedicated time slot.
Polling techniques have also been employed in the prior art to address this problem. Under a polling technique, no mobile station transfers any message to the base station unless and until it receives a message from the base station asking for information. Therefore, polling also results in inefficient use of a radio data communications link because every communication between a given mobile station and the base station must be initiated by a polling message from the base station.
Contention protocols are another known technique used to address this problem. In a contention protocol, each time a mobile station sends a radio data message to the base station, the mobile station expects an acknowledgment from the base station indicating successful receipt of its message. If the mobile station does not receive this acknowledgement, the mobile station repeatedly re-transmits its message after a random time delay until acknowledgement is received. A failure to receive an acknowledgment from the base station is typically due to (1) overlap between multiple radio data messages which arrive at the base station at approximately the same time, (2) the mobile station being out of radio range of the base station, or (3) the mobile station being in a "radio valley or hole." Re-transmissions due to cause (1) often create problems in known contention protocols. As the number of messages to be transmitted between the mobile stations and the base station increases, the frequency of re-transmissions necessitated by overlapping messages increases, and the availability of the radio data communications link decreases in a cascading manner.
Finally, in environments in which mobile stations are required to send both routine position updates and also non-routine, priority messages to a base station, existing systems have employed the technique of having each individual mobile station transmit its position update to the base station only when it has traveled a fixed, radial distance since it last transmitted a position update. Assuming that the volume of the routine position updates accounts for a sufficiently large proportion of the total data messages transmitted between the mobile stations and the base station, controlling the volume of the routine position updates can increase overall radio data communications link availability. However, this technique results in inefficient use of a radio data communications link when the link has sufficient availability to handle more frequent position updates, and this technique does not ensure communications link availability during high volumes of non-routine messages.
It is therefore an object of the present invention to provide an improved system for ensuring the availability of a radio data communications link for priority radio data messages between a fleet of mobile stations and a fixed base station.
It is a further object of the present invention to provide such a system for operation in an environment in which routine data messages and non-routine, priority data messages are sent from the mobile stations to the base station.
It is a further object of the present invention to provide such radio data communications link availability by dynamically altering the frequency of the routine data messages from the mobile stations to the base station in response to the number of data messages requiring multiple transmissions for successful receipt by the base station.
It is a further object of the present invention to provide such radio data communications link availability by dynamically altering the frequency of the routine data messages from the mobile stations to the base station in response to the number of data messages successfully received by the base station.
It is a further object of the present invention to optimize the overall use of the radio data communication link and to achieve a more uniform flow of radio data messages.
Still other objects and advantages of the present invention will become apparent to those of ordinary skill in the art having references to the following specification together with its drawings.